Process of desulfurization employing alkali and nitrogen dioxide



United States Patent 3 267 027 PROCESS OF DESUIJFUIQIZATHON EMPLOYINGALKALI AND NITROGEN DIOXIDE William L. Fierce, Crystal Lake, and WalterJ. Sandner, Carpentersville, IllL, assignors, by mesne assignments, toUlllOll Oil Company of California, Los Angeles, Calif., a corporation ofCalifornia No Drawing. Filed July 17, 1961, Ser. No. 124,352 4 Claims.(Cl. 208-228) This invention relates to new and useful improvements inprocesses for the refining of light petroleum distillates which containa substantial quantity of malodorous sulfur compounds. Moreparticularly, this invention is concerned with a process for deodorizingand desulfurizing light petroleum distillates by treatment with nitrogendioxide at about 035 C., followed by successive washing with aqueouscaustic and water.

Petroleum fractions, e.g., naphthas, kerosines, gasolines, fuel oil,lubricating oil, waxes, etc., as normally obtained from petroleum oftenhave unsatisfactory odor characteristics resulting from the presence ofmalodorous sulfur compounds of various types. The removal of thesemalodorous constituents has often proved diflicult by ordinary refiningprocedures. In the past, the removal of mercaptans has been accomplishedto some extent by clay treating and by treatment with chemicals whichform readily extractable derivatives. In some cases, the removal ofsulfur compounds has been impractical and the sour petroleum fractionshave been treated by a sweetening process in which mercaptans areconverted to disulfides (which have less objectionable odors). A processof this type, however, does not remove sulfur from the product and mayresult in a product which is undesirable even though free of odor ofmercaptans.

In the past, petroleum fractions have been treated with a variety ofchemicals for producing products with substantially improved odors.Naphthas have been treated with adsorbents of various types, and havebeen treated with various amines during distillation to preventformation of malodorous constituents. Petroleum fractions have beensubjected to solvent extraction with various acidic materials such assulfur dioxide, phenol, and concentrated acids to extract color-formingand odor-forming constituents. The processes of the prior art, however,have been expensive and difficult to carry out and have not alwaysproduced products of good odor characteristics which are free of sulfurcompounds.

It is therefore one object of this invention to provide a process forthe treatment of light petroleum distillates which results in theelimination of mercaptans, reduction in content of other sulfurcompounds, and a substantial improvement of odor.

A feature of this invention is the provision of a process for thetreatment of light petroleum distillates with nitrogen dioxide at about035 C., followed by washing of the treated distillate with aqueouscaustic and water.

Other objects and features of this invention will become apparent fromtime to time throughout the specification and claims as 'hereinafterrelated.

In measuring the relative odor of petroleum products it has not beenpossible to develop a precise, quantitative measure of odor which isentirely independent of the individual who evaluates the odor.Nevertheless, there are certain procedures for rating odors which havebeen used in the petroleum industry. Odor evaluation standards andmethods have been proposed by a joint committee of the ASTM and TA'PP-I.A modification of this procedure has been used extensively in theevaluation of the several, e.g., to 20 or more, individuals who have"ice been tested for their ability to discriminate between and matchdifferent odors. One test for selection of panel members involvescorrectly matching a series of chemical odors such as very dilute odorsof acetic acid, phenol, toluene, and benzene. A second test involves amatching of a variety of different naphthas by odor from a singlemanufacturing source. A third test involves the matching of mineralspirits obtained from different man-ufacturers. By use of thesescreening tests it is possible to select a panel of 10 to 20 individualswho are especially discriminating in evaluating odors of petroleum products. In the rating of odors of petroleum fractions such as an odorlessnaphtha, a numerical rating scale has been established to describe odorquality.

Numerical rating: Odor quality 1 Excellent.

2 Excellent.

3 Very good.

4 Very good.

5 Good.

6 Fair (just passing). 7 i Borderline.

8 Borderline.

9 Poor.

10 Very poor.

When evaluating the odor of petroleum naphthas the following procedureis used. The members of the odor panel are requested to avoid contactwith contaminating odors, e.g., smoking, etc., for at least /2 hourprior to the test. The odor evaluation is carried out in anairconditioned room, at about 24 C., which is as free as possible fromextraneous odors. Each member of the test panel rates the odor of aproduct separately. A nonmember of a panel conducts the test and ispresent to record any pertinent comments made regarding the odor. Toeliminate bias, the product samples are coded, and the panel members aretold the type of product being rated. To avoid odor fatigue, no morethan three samples are rated at any one time. Any additional ratings arespaced by at least three hours. A total of seven panel members rate theproduct and the average of these opinions is reported as the odorrating.

In preparing the naphtha for odor evaluation, a 35-rnl. representativeportion is poured into a clean, odor-free, eight-ounce French squarebottle. The bottle is sealed with a clean, odor-free, screw-on cap. Thesame sample is used by all members of the panel. The type bottle used inthe evaluation is the same as that used in the screening tests forselection of the panel member. The panel member evaluates the odor ofthe naphtha by removing the cap from the bottle, placing his nose at thebottle mouth and sniffing the odor. He checks the intensity of odor andfor any foreign or undesirable odor that may be present, and thennumerically rates the odor using the above-described scale. This methodis called the wet-odor of the naphtha.

Sulfur compounds, such as mercaptans, sulfides, and disulfides, areoften present in petroleum fractions. In many cases, it is necessary toremove a substantial portion of the sulfur compounds in order to producepetroleum products of high quality. We have devised a process for thereduction of sulfur content and improvement in color and odor of lightpetroleum distillates. Our process consists essentially of the followingsteps: (1) A sour light petroleum distillate is treated with gaseousnitrogen dioxide at a low temperature in the range from about 0-35 C.The nitrogen dioxide may be charged alone or in admixture with inertgaseous diluents such as nitrogen, hydrogen, helium, etc. In general,the amount of nitrogen dioxide should be at least equal to thestoichiometric amount required to react with all of the sulfur compoundsin the petroleum distillate being treated. However, because the amountsof each of the types of sulfur compound present are dilficult todetermine, and because all of the reactions which occur in this processhave not been determined, the amount of nitrogen dioxide required isbest determined by a screening test treatment of a small sample of thedistillate. The product from the first step may be yellow in color. (2)The treated petroleum fraction is washed with aqueous caustic until allof the yellow color has been removed. (3) The product is finally washedwith water until all alkaline material has been removed, and the productmay be dried if necessary.

In carrying out this invention we have found that all three steps of ourprocess are essential to obtaining a mercaptan-free, colorless productof diminished sulfur content and improved odor. When any of theabove-noted steps of treatment are eliminated, an unsatisfactory productis obtained. When the product is treated with nitrogen dioxide andwashed only with water, the product tends to have an unsatisfactoryyellow color. When the treatment with nitrogen dioxide is omitted, theyellow color does not occur but there is no appreciable reduction inodor or sulfur content, whether washed with water or caustic. When thetemperature of the N treatment exceeds 35 C., there is an undesirableoxidation of hydrocarbon constituents which may produce additionalmalodorous compounds. The mechanism by which the process operates tosweeten and reduce the sulfur content of light distillates is not knownprecisely. The treatment probably involves the conversion of mercaptansto disulfides, and the conversion of disulfides and monosulfides tosulfoxides or sulfones, or to unknown derivatives which are somehowrendered soluble by treatment with aqueous caustic. Whatever theidentity of the reaction products and the mechanism of the reactions,the net result is that the sulfur compounds are converted to productswhich can be readily separated from the petroleum fraction by washingwith water and aqueous caustic. We have no explanation for the yellowcolor produced by the nitrogen dioxide treatment. The material whichcauses this col-or is not water-soluble and the material which producesthe color is unknown in composition. The removal of the color by causticwashing may be due to solubility of the colored material in aqueouscaustic or the colored material may be converted by the caustic to awater-soluble material which is removed.

The following non-limiting examples are illustrative of the scope ofthis invention,

Example I A light petroleum distillate (a naphtha) having thedistillation range: I.B.P. 44 C., 5% 58 C., 50% 127 C., 95% 200 C., andBF. 212 C., was treated partially in accordance with this invention.This naphtha was evaluated sour by the doctor test and had an odorrating of 9. It had a tot-a1 sulfur content of 0.087% wt., most of whichconsisted of mercaptans, sulfides, and disulfides. In this experiment a500-ml. 3-necked flask was fitted with a fritted gas bubbler, a stirrer,a thermometer, and a condenser. The flask was cooled with an ice-waterbath and charged with 300 ml. of the sour naphtha. Nitrogen at 30cc./min. and gaseous nitrogen dioxide at 179 cc./min. were bubbledthrough the naphtha for 30 minutes, with stirring. A total of 10.1 g. ofnitrogen dioxide was charged. Next, the naphtha was placed in a 500-cc.separatory funnel and washed six times with 200 ml. portions of water.The final water wash was neutral as measured by pH paper. The naphthawas dried by filtering it through a double thickness of filter paper.The final product was doctor sweet, had a total sulfur content of 0.021%wt., and had a. fairly pleasant odor (odor No. about 3), but was brightyellow in color.

Example II In another experiment, a fresh 300-ml. batch of the sournaphtha used in Example I was charged to the apparatus described inExample I. In this experiment, nitrogen was bubbled through the naphthaat 60 cc./min. for 30 minutes, with stirring, without any nitrogendioxide being added. Then the naphtha was washed six times with waterand filtered through a drying paper (Reeve Angel paper). The productwhich was obtained in this experiment was colorless but was definitelysour, had an odor rating of about 78, and a sulfur content of about0.083% wt.

Example III In another experiment, a fresh 300-ml. batch of the sournaphtha used in Examples I and II was charged to the apparatus describedabove. Nitrogen at 30 cc./min. and gaseous nitrogen dioxide at 5 3 cc./min. were bubbled through the naphtha for 10 minutes, with stirring. Atthis point, a slight yellow-brown color began to appear above theliquid. A total of 1.0 g. of nitrogen dioxide was charged. The treatednaphtha was then washed five times with water and the final wash wasneutral to pH paper. The treated naphtha was filtered and storedovernight. The color of the product was a clear, bright yellow. The nextday the odor of the naphtha was found to be slightly acidic andcharacteristic of the oxides of nitrogen. The treated naphtha was thenwashed with four SO-ml. batches of 0.4 N sodium hydroxide. Thistreatment removed both the color and the acidic odor. Finally, thenaphtha was washed four times with water and dried by filtering asbefore. The product was doctor sweet, had an odor No. of 2, and wasreduced about 50% in total sulfur content, substantially all of themercaptans having disappeared from the product.

Example IV In still another experiment, a fresh 300-ml. batch of theoriginal sour naphtha was washed with four 50-ml. batches of 0.4 Nsodium hydroxide without receiving the nitrogen dioxide treatment. Theproduct was then washed four times with water and dried by filtering.The product was strongly doctor sour, although the odor was slightlyimproved over the original naphtha.

From our experiments we have found that this treatment is applicablebroadly to light petroleum distillates only, i.e. disti'llates boilingin the range from 38 to 288 C. When a petroleum distillate boiling inthis range is treated with nitrogen dioxide, with or without a diluentgas, the sulfur compounds are converted to a form which can :beextracted with aqueous alkali. The nitrogen dioxide must be added in anamount not less than the stoichiometric amount required to react withall of the sulfur compounds in the distillate, preferably 1 to 10 g.nitrogen dioxide per gram of sulfur in the material being treated. Thetreatment with nitrogen dioxide must be carried out at relatively lowtemperatures, preferably in the range of 0 to 35 C., since the use ofhigher temperatures causes the nitrogen dioxide to attack some of thehydrocarbons in the distillate. The treatment is carried out for a timesufficient to effect complete reaction with the sulfur compounds,preferably in the range of 5 to 30 minutes. The light distillate whichhas been treated with nitrogen dioxide is washed with aqueous alkali,preferably 0.1 to 2.0 N sodium hydroxide, although other alkali such aspotassium hydroxide, lithium hydroxide, or ammonium hydroxide may beused. The final wash with water is carried out for a time suificient torender the product free from alkali.

The combination of steps used in this process are effective to removesubstantially all of the mercaptans from the distillate, as well as amajor portion of other sulfur compounds, while the treatment with alkaliremoves the undesirable color-forming materials which are producedduring the nitrogen dioxide treatment. While the treatment has beendescribed specifically as applied to a sour naphtha, it is applicable toother distillates boiling in the range of 38 to 288 C., includinggasolines, light naphthas, medium naphthas, kerosine, and lighthydrocarbons, such as heptane, benzene, toluene, etc., which containmalodorous sulfur compounds. In each case, the treatment is effective toremove substantially all of the mercaptans as well as a major portion ofother sulfur compounds, while rendering the product doctor sweet. Theproduct which is finally recovered from the process is not only doctorsweet, but is improved in odor from an initial rating in the range ofabout 9-10 to a rating in the range of about 1-3.

While we have described our invention fully and completely with specialemphasis upon several preferred embodiments, we wish it to be understoodthat within the scope of the appended claims this invention may bepracticed otherwise than as specifically described herein.

The embodiments of the invention in which an exclusive property orprivilege is claimed are as follows:

1. A method of producing petroleum products of diminished sulfur contentand improved odor which consists essentially of contacting a sour lightnaphtha with nitrogen dioxide at about 0-35 C., washing the treateddistillate with aqueous alkali until free of color, and washing thecolor-free product with water until free of alkali.

2. A method in accordance with claim 1 in which the nitrogen dioxide isused in an amount not less than the stoichiometric amount required toreact with all of the sulfur compounds in the naphtha.

3. A method of deodorizing and desulfurizing light naphthas whichconsists essentially of contacting a sour light naphtha with suflicientgaseous nitrogen dioxide, at about 035 C., to react with substantiallyall of the sulfur compounds in the naphtha, washing the treated naphthawith a 0.4 N solution of NaOH in water until free of color, and washingthe color-free product until neutral.

4. A method in accordance with claim 3 in which the naphtha is driedafter the final Washing.

References Cited by the Examiner OTHER REFERENCES Kalichevsky et al.:Petroleum Refining With Chemicals, 1956; pp. 138-141 relied on; ElsevierPub. Co., New York.

DELBERT E. GANTZ, Primary Examiner.

ALPHONSO D. SULLIVAN, MILTON STERMAN,

Examiners. S. P. JONES, Assistant Examiner.

1. A METHOD OF PRODUCING PETROLEUM PRODUCTS OF DIMINISHED SULFUR CONTENTAND IMPROVED ODOR WHICH CONSISTS ESSENTIALLY OF CONTACTING A SOUR LIGHTNAPHTHA WITH NITROGEN DIOXIDE AT ABOUT 0*-35*C., WASHING THE TREATEDDISTILLATE WITH AQUEOUS ALKALI UNTIL FREE OF COLOR, AND WASHING THECOLOR-FREE PRODUCT WITH WATER UNTIL FREE OF ALKALI.